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Butter and cheese may have suffered a bad rep for decades partly because a group of scientists didn’t report all their data.

A major cholesterol study from the 1970s, part of the evidence that drove consumers to vegetable oil and margarine, doesn’t actually show all that the authors claimed.

Now the British Medical Journal says in an editorial that a fresh analysis of the original data challenges the “established wisdom” about switching from saturated (animal) fats to unsaturated vegetable fats, because this “might not prolong life” after all.

The episode shines a light onto what can happen when a scientific study doesn’t really show what everyone thinks it does.

U.S. scientists selected 9,423 longterm residents of state mental hospitals and one nursing home, and they became participants in a multi-year health study. Some were switched from saturated fats to more corn oil, margarine, egg substitutes, processed cheese and whipped cream substitutes, and others were not. Both groups, by today’s standards, were eating too much fat of one kind or another.

The study, which ended in 1973 but was only published in 1989, found that the group eating vegetable oil lowered their levels of cholesterol and may have had less heart disease. It is known today as the Minnesota Coronary Survey.

But in the decades since, there were lingering questions. Lower cholesterol from vegetable oil — yes, that happened. But a connection with healthier hearts? Longer lives? Proving this link turned out to be elusive.

Finally a team from the National Institutes of Health and University of North Carolina went back to analyze the Minnesota data again. The original published study left out vital data, they write in this week’s British Medical Journal, and “incomplete publication has contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in linoleic acid.” It suggests there may even have been more deaths from heart disease in the low-cholesterol group, caused by too much polyunsaturated fat.

Modern researchers often post all their raw data online, and it’s partly to avoid leaving things out, says John Smol, a leading Canadian biologist and journal editor.

“In science there are blatant lies — that’s people fabricating data. And now you get into another area which is serious (which) is not presenting all the data. One could use the word cherry-picking. You decide in advance what you think the answer must be, or what you convince yourself the answer is, and you tend to slant the data,” he said.

“That’s definitely not the way science is supposed to go either,” and that is why studies are “peer-reviewed,” or double-checked by outside experts.

So what should we eat? The debate continues: The Canada Food Guide wants us to avoid saturated fats and to choose small amounts of unsaturates. The BMJ now suggests that preferring unsaturates may lower cholesterol without saving lives. And recently the U.S. government stopped warning people against eggs, ruling that the dietary cholesterol in them isn’t dangerous after all.

In the meantime, the BMJ’s closing advice may sound like your mother’s: “We should continue to eat … more fish, fruits, vegetables, and whole grains. We should avoid salt, sugar, industrial trans fats, and avoid over eating.”

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Butter and cheese are better than you think: Why the general wisdom on cholesterol could be wrong

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